This invention relates to improved apparatus and methods for making tubing from sheet metal by shaping the metal sheet into the form of a tube with an oval-shaped cross section and a longitudinally extending seam, which is welded to form a closed tube. Thereafter, the oval-shaped tube may be deformed to have a circular cross section. By starting with a metal sheet in the shape of a trapezoid, tapered tubing may be formed. Sections of tapered tubing welding end-to-end make strong, lightweight, tapered poles useful as supports for traffic lights, street and highway lights, signs, power lines, and the like.
It has long been the practice to make sheet metal tubing of uniform or tapered diameter by starting with a strip of sheet metal and using forming rolls to shape the sheet into tubular form of oval cross section and with a longitudinal seam line which may be welded. For example, U.S. Pat. No. 3,452,424, issued July 1, 1969, to J. Morris for "Forming and Welding Tapered Tubes", discloses making tapered tubes from trapezoidal metal sheets. Two disadvantages of this prior art method and apparatus are:
(1) the edges to be welded together are deformed by edging rolls before the sheet is bent around the longitudinal axis of the sheet, which requires complex controls for the edging rolls to handle the trapezoidal shape of the metal sheet, and which can result in improper forming and positioning of the edges with respect to each other for welding; and
(2) the work is supported on an adjustable roller as it passes a welding station, and is subject to imprecise positioning and support during the welding operation.
U.S. Pat. No. 3,253,452, issued May 31, 1966, to Scott for "Method and Apparatus for Forming Elongated Tubular Tapers" also discloses deforming a trapezoidal sheet of metal into a tapered tube with a noncircular cross section and a longitudinally extending seam line. Before welding the adjacent portions of the tube along the seam line, the Scott procedure requires deforming the tube into a circular cross section and then welding the seam. Although this method has produced satisfactory tubing, it is relatively slow and requires special handling to be sure that the unwelded seam of the round tube is in proper positioning for welding. Moreover, the prior art processes did not provide a good match in the outside diameters of adjacent ends of sections of tapered tubing welded together to form a pole. This was especially true when sheet metal of different gauges were used to try to save material.
This invention provides improved methods and apparatus for forming, efficiently and precisely, tubes of different shapes and sizes from pieces of sheet metal of different gauges, and which can be welded together to form attractive and structurally sound poles.
The apparatus can be quickly modified to produce cylindrical or tapered tubes of different cross sectional areas and shapes. Moreover, the longitudinal edges of the tube to be welded together are precisely formed and positioned with respect to each other to ensure precision and satisfactory welding of each tube produced in accordance with this invention.
In terms of method for forming a tube from a metal sheet having at least one pair of opposite side edges, the invention includes the steps of deforming the metal sheet around a central longitudinal axis to move the side edges toward each other until the sheet is in the shape of an elongated trough. Thereafter, the side edges are each deformed about a separate, respective side longitudinal axis until the sheet is converted from a trough to a tube having an oval cross section and an elongated seam where the opposing side edges are adjacent each other. The adjacent edges of the sheet are welded together to form a closed tube. If the final product is to have a circular cross section, the oval-shaped tube is thereafter disposed between a pair of spaced-apart, opposed, and substantially semicircular die cavities. The two die cavities are moved toward each other to cold-work and deform the oval-shaped tube into one of circular cross section.
In terms of apparatus, the invention includes an elongated track and an elongated wagon open at its top and mounted to roll on the track. The wagon has an upwardly facing bottom which is curved concave upwardly about a longitudinally extending axis to form a U-shaped trough in the bottom of the wagon. Means are provided for moving the wagon along the track. Means are also provided for supporting a metal sheet over the trough in the wagon at a U-ing station, and for deforming the metal sheet to fit in the trough and leave opposite side edges of the metal sheet projecting up from the wagon. At least one deforming roller over the track engages the side edges of the sheet projecting from the wagon (as the wagon moves along the track), and deforms the side edges toward each other to form a tube with an oval-shaped cross section and an elongated seam where the side edges are adjacent each other. Electrical welding means over the track engages the deformed side edges of the metal sheet as the wagon moves, and heats the edges to welding temperature. Forging rollers mounted over the track and adjacent the welding means drive the heated side edges of the strip together and form a closed tube with a welded seam extending for substantially the length of the tube.
Preferably, the welded tube is oval-shaped in cross section, and means, such as an O-ing machine, are provided for squeezing it into a substantially circular cross section.
For forming tapered tubes, the metal sheet is of trapezoidal shape, and the longitudinal axis of the bottom of the trough in the wagon is inclined with respect to the direction of the wagon travel on the track. In the preferred form, a removable die box with a sloping bottom fits inside the wagon to form the trough with the bottom inclined with respect to the direction of wagon travel. Spacers are preferably provided for fitting under the die box so that the height of the die box can be adjusted to accommodate metal sheets of different sizes.
The means for deforming the side edges of the sheet to close the tube and form the seam is preferably a fin roller.